Recently, with the increasing demands for miniaturization, weight reduction, multifunctionalization and speed-up of electronic devices, highly accumulated, micronized electronic parts are required in quantity and at the same time, demands for highly dense, reliable printed circuit boards are increasing day by day. In the preparation of a printed circuit board, a method has been used wherein a resist film pattern corresponding to the desired circuit is formed on a copper layer on an insulation substrate either by a direct printing or a combination of steps of applying on the copper layer a photosensitive resin coating, exposing the coating, through a circuit pattern mask placed thereon, to an actinic light and developing the same, and subjecting the thus obtained plate having the resist film pattern thereon to an etching to obtain the desired copper circuit pattern. At the present day, as a pattern forming method, either of such screen printing method and photographic method with a dry film are generally used. However, both methods have several problems with respect to fineness and productivity. More specifically, in a screen printing method, it is hardly possible to have a fine pattern, and in a dry film photographic method, there are problems of poor adhesion to copper, inferior coverage for irregular plate surface derived from uneven polishing and the like. Therefore, conventional coating methods of using a liquid resist material by dipping, roller coating, rotary coating and spray coating are now being considered again. However, there still remains problems in respect of uneveness of the formed coating, pinholes, uneven coating thickness and the like.
For the image-formation on both surfaces of a substrate or a multi-layered plate, a substrate having through-holes previously coated with copper by an electroless plating or electroplating is usually used. During an etching operation, said copper coat on the through-hole surface must be fully protected for securing the roles of such through-hole as electronic parts insertion hole and electric conduction passage. For this purpose, the so-called tenting method in which the through-holes are covered by a dry-film or hole-filling method in which the through-holes are previously filled with a resist material is heretofore used prior to the formation of a resist pattern with a liquid resist composition. Among them, the tenting method is most widely used because of the least number of operational steps. However, this cannot give satisfactory results with a substrate having a number of mini-via-holes with very narrow lands.
To cope with the same, an electrodeposition coating method now becomes the object of public attention. Heretofore proposed photosensitive resin compositions for resist film are classified into two groups, i.e. one is a negative type photosensitive resin composition (the so-called photocuring type photosensitive resin composition) and the other is a positive type photosensitive resin composition (the so-called photosolubilizing type photosensitive resin composition). When the former is used for the formation of photosensitive resist film, thus obtained resist film is photocured at the exposed area and unexposed portions are subsequently removed off by an appropriate solvent in a developing step, thereby leaving the photocured resist corresponding to the desired circuit pattern. Such methods and materials are disclosed in many publications as, for example, Japanese Patent Publication (unexamined) Nos. 247090/86, 262855/87, 262856/87, U.S. Pat. No. 4,592,816 and the like. The materials disclosed, in general, contain, as main ingredients, a saturated or unsaturated polymer containing ionic groups, a photopolymerization initiator and optional unsaturated monomers. Such materials show good adhesion toward copper coating, exhibit higher coverage for uneven surface due to their slowing power properties and can be applied automatically and on continuous basis, without the necessity for using expensive installation devices and excessive man power.
However, in order to obtain a good film which is well resistive toward the actions from a developer and an etching solution, in a possible short exposure time, it is generally required to add a liquid or semi-solid unsaturated monomer. This will, in turn, result in a tacky coating. Therefore, when a circuit pattern mask is placed on the resist film and the composite is exposed to an actinic light, undesired adhesion of uncured resin to the mask often occurs and repeating use of the same mask is interrupted. In this process, it is, therefore, required to provide an adhesion proof layer on the formed resist film and hence, the process is uneconomical. Furthermore, for a better etching resist, the applied resin must be photocured even at a narrow inner surface of the through-hole. Nevertheless, for mini-via-holes with considerably large aspect ratio, a longer exposure time is required for the desired photocuring. Therefore, if the exposure time agrees with the time required for the photocuring of the resin located at the inside of narrow through-holes, excessive photocuring occurs at the plane resist surface, causing image gain of the resist pattern. Moreover, it is almost impossible to cause an effective photocuring at the inside of through-holes having an average diameter of 0.3 mm or less.
In contrast thereto, a positive type photosensitive resin composition is totally free from the abovementioned problems. Examples of such positive type photosensitive resin compositions for electrodeposition coating are aqueous dispersion of phenolic novolac resin having in its phenol nucleus both a quinonediazido group and a hydrophilic ionic group as disclosed in Japanese Patent Publication (unexamined) No. 207139/85, and aqueous dispersion of modified phenol resin having both a quinonediazido group and an acidic group as disclosed in Japanese Patent Publication (unexamined) No. 218616/86. Such compositions are useful in the electrodeposition coating for complicated structural materials as a circuit board with through-holes, and are capable of forming a uniform coating with good adhesion toward copper. However, when applied in a thin layer, occurrence of pinholes in the formed coating is unavoidable due to the generation of gas through an electrode reaction. Since the phenolic novolac resin is brittle by nature and has inferior thermal flowability, restoration of the pinhole area is very difficult to do. Therefore, many defects often occur in the circuit pattern and hence such compositions are inadequate for the resist film to be used in the formation of fine circuit pattern.
The inventors have previously proposed, under the circumstances, a positive type photosensitive resin composition comprising a quinone diazido compound and an acrylic resin bearing at least one side chain having the structure of ##STR1## wherein R.sub.2 is a substituted or unsubstituted hydrocarbon residue selected from aliphatic hydrocarbons having 2 to 10 carbon atoms, alicyclic hydrocarbons having 6 to 7 carbon atoms and aromatic hydrocarbons having 6 carbon atoms; A is a repeating unit of the formula: ##STR2## (in which R.sub.3 is ethylene or propylene; R.sub.4 is a substituted or unsubstituted alkylene having 2 to 7 carbon atoms; R.sub.5 is a substituted or unsubstituted alkylene having 2 to 5 carbon atoms; k and l each represents an average repeating unit number and k is 1 to 10 and l is 2 to 50); or ##STR3## wherein R.sub.7 is a substituted or unsubstituted hydrocarbon residue selected from aliphatic hydrocarbons having 5 to 10 carbon atoms, alicyclic hydrocarbons having 6 to 7 carbon atoms and aromatic hydrocarbons having 6 to 13 carbon atoms; R.sub.8 is selected from aliphatic hydrocarbons having 1 to 30 carbon atoms or aromatic hydrocarbons having 6 to 13 carbon atoms optionally substituted with any one of a vinyl group, allyl group, ether group, ester group, and carbonyl group, each singularly or in combination of two or more in the main chain, and optionally substituted in the side chain(s); B is a ##STR4## (in which m and n each is 0 or 1); D is a repeating unit of the formula: ##STR5## (in which R.sub.9 is ethylene or propylene; R.sub.10 is a substituted or unsubstituted alkylene having 2 to 7 carbon atoms; R.sub.11 is a substituted or unsubstituted alkylene having 2 to 5 carbon atoms; p and q each is an average repeating unit number and p is 1 to 10 and q is 2 to 50), and E is a carboxylic acid, sulfonic acid, phosphoric acid or phosphorus acid group., (PCT/JP88/00093, filed Sep. 30, 1988; EP Appln. 88 901 323.1, published Feb. 1, 1989 as A1 0301101). The present inventors have also previously proposed a positive type photosensitive resin composition comprising a resin obtained by the reaction of (a) at least one polyepoxide compound having an epoxy equivalent of 75 to 1,000, selected from the group consisting of polyvalent alcohol glycidyl ester compound, polycarboxylic acid glycidyl ester compound, alicyclic glycidyl compound, glycidyl amine compound, heterocyclic glycidyl compound and bisphenol-A alkylene oxide compound, with (b) aromatic or heterocyclic carboxylic acid bearing phenolic hydroxyl groups represented by the formula: ##STR6## in which A is a substituted or unsubstituted alkylene or arylene, or ##STR7## R.sub.4 and R.sub.5 each represents a substituted or unsubstituted alkylene or arylene; n is 0 or 1; m is an integer of 1 to 3; R.sub.2 and R.sub.3 each represents hydrogen, halogen, alkyl, alkenyl, nitro or alkoxy group, and R.sub.2 and R.sub.3, taken together with carbon atoms of the benzene ring may form an aromatic, alicyclic or heterocyclic ring, and (c) 1,2-quinonediazido sulfonic acid halide, in the ratio, in terms of equivalent ratio, of carboxyl group of said (d): epoxy group of said (c)=1:0.8 to 1.2 and phenolic hydroxyl group of said (d): sulfonyl halide group of said (e)=1:0.05 to 1.2, (PCT/JP88/00094, filed Feb. 2, 1988; EP Application 88901324.9, published Feb. 15, 1989 as A.sub.1 0302941).
These compositions are excellent because they result in coatings with excellent flexibility and adhesion, whose unexposed area will give the least swelling at the development stage. However, there still remains a question of the stability of a sensitive group of the compositions.
It is therefore, an object of the invention to provide a positive type photosensitive resin composition which is excellent in storage stability of the photosensitive group, is capable of resulting in a coating, by electrodeposition means on various base plates, with excellent flexibility and adhesion to copper, the coating being free, even in a thin layer, from pinholes and highly sensitive, excellent in alkaline development and affording the least undesired swelling, if not exposed, at the developing stage and hence specifically useful as a resist for the formation of a fine circuit pattern. It is an additional object of the invention to provide an image-forming method specifically useful for the formation of a fine circuit pattern with a substrate having mini-via-holes.